Although, as just previously mentioned, the invention is not in its broad aspect necessarily confined to the examination of bank-notes, it is convenient to outline the problems giving rise to the present invention with respect to that particular application of the invention, since the existing problems which the present invention is intended to solve or at least alleviate are particularly acute in relation to bank-notes.
Bank-notes are printed by a variety of printing processes which are necessarily complex. Bank-notes are not necessarily exactly the same yet the implementation of the limits of acceptability may vary according to the individual examiner. Owing to the inherent variations of printing processes, it is, as will be further explained hereinafter, particularly difficult to mechanise the process of examination owing to the difficulty of formulating an absolute criterion of acceptability of a bank-note using, for example, a master note which is agreed to constitute a perfect specimen. Another quite general problem is the variation even over the area of a bank-note of a criterion of acceptability. For example, what may be reckoned to be unacceptable in one region of a bank-note may be acceptable in another. One convenient example is that a particular imperfection occurring in the face of a Head of State or other person represented on a bank-note may reasonably be regarded as unacceptable whereas an imperfection of the same order of magnitude but occurring in some border region of the bank-note may be regarded as an acceptable imperfection. The variation in a criterion of acceptability may also occur in relation to, for example, industrial components of which particular dimensions must be made to a smaller tolerance than is acceptable in other dimensions.
A further difficulty in the development of a machine technique for the examination of bank-notes lies in the variability of the media which are used for their production. The paper may shrink or stretch and the printing medium, for example ink, may vary in consistency or density. All these factors produce minor variations which may in general be tolerable to the human eye, depending on where they occur, but represent severe difficulties in the development of a technique for examination of a bank-note by automatic means. In general, of which the foregoing are particular examples, if a machine is capable, as has been thought necessary hitherto, of resolving the finest detail in the bank-note then the more prone it is to the indication of imperfection even when imperfection is tolerable.
These various problems are accentuated if any attempt is made to examine by machine a large number of bank-notes individually and in particular in which is usually termed an "on-line" manner, that is to say when the examination of a bank-note is to be performed within the time interval between the presentation of successive bank-notes at some stage in the cycle of production thereof. The information content of a bank-note examined to a reasonable degree of resolution is very considerable and the development of a machine technique for the examination of bank-notes for acceptability is made difficult by the very high information rate at which a machine must operate if it is to be used "on-line".